High temperature fuel cells like solid oxide fuel cells comprise an electrolyte sandwiched between a cathode and an anode. Oxygen combines with electrons at the cathode to form oxygen ions, which are conducted through the ion-conducting ceramic electrolyte to the anode. At the anode, oxygen ions combine with hydrogen and carbon monoxide to form water and carbon dioxide thereby liberating electrons.
The fuel cells are stacked and interleaved with interconnect plates which distribute gases to the electrode surfaces and which act as current collectors. Contact pastes are used to bond the electrode to an interconnect and must therefore be electrically conductive. In co-owned U.S. Pat. No. 6,420,064, a cathode contact layer comprised of lanthanum cobaltate is disclosed.
Lanthanum cobaltate (“LC”) (also known as lanthanum cobaltite) is a perovskite material, which is a well-known class of mineral oxides characterized by a cubic or orthorhombic crystalline structure. Perovskites may be described by the formula ABO3, where A represents divalent and/or trivalent ions and B represent trivalent and/or tetravalent ions, respectively, while the O atom is the oxygen ion. The divalent, trivalent and tetravalent ions may include La3+, Sm3+, Sr2+, Ca2+, Co3+, Ni3+, Fe3+, Cr3+, Mn3+ or Mn4+ amongst other known ions. In cubic perovskites, this ABO3 structure in a general sense can be thought of as face centered cubic (FCC) lattice with A atoms at the corners and the O atoms on the faces. The B atom completes the picture and is located at the center of the lattice.
Some perovskites such as LC are reasonably good electrical conductors. However, as a contact paste in a Ni-YSZ anode-supported SOFCs, LC suffers from one significant disadvantage. If sintered, its coefficient of thermal expansion is significantly greater than that of the bulk cell. Consequently, thermal cycling of the fuel cell results in large thermal stresses and the contact paste may break away from the cell and interconnect resulting in poor electrical contact.
In some cases, contact paste materials which display better interface performance with the cell can have poor interface performance with the interconnect.
Therefore, there is a need in the art for fuel cells having an improved contact paste with a multilayer design which is electrically conductive and which mitigates the difficulties in the prior art.